This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of porcine reproductive and respiratory syndrome, which is the most severe infectious disease facing the swine industry worldwide. PRRSV is highly host-restricted, growing preferentially in vivo in porcine macrophages. In vitro, PRRSV grows in primary cultures of porcine alveolar macrophages, as well as in MARC-145 cells, the African green monkey kidney epithelial-like cell line, which have been routinely used for in vitro propagation of wild-type and vaccine strains of PRRSV. However, between the two types of host cells, a certain PRRSV strain usually has a preference on one cell type over the other. Understanding the molecular structure of PRRSV is expected to facilitate the study of cell tropism and the development of disease prevention strategies. PRRSV has a smooth spherical envelope embedded with major envelope proteins (GP5/M heterodimer) and minor envelope proteins (E, GP2a, GP3, GP4), most of which are highly glycosylated. The molecular weight of GP5-linked glycans exceeds that of its short ectodomain amino acid sequences. Thus the broadly distributed viral glycans are likely to cover the virion surface, or even stretch out as antennae, suggesting a potential role in virus-cell interactions and possible targets for virus neutralization in microbicide design.